Vitamin D: Testing, Repletion & Why It Matters for Skin & Immunity

Written by Empire Medical Training Editorial Team

Medically reviewed by Mark Tager, MD Integrative & Functional Medicine Physician; Author, ‘Feed Your Skin’

Updated June 2026 ·11 min read

Vitamin D sits at an unusual crossroads in clinical nutrition. It is classed as a vitamin, but the body actually synthesizes it like a hormone, and its main source is not the plate but the sun. That single fact — that a “nutrient” depends largely on sunlight — explains why insufficiency is so widespread, why it shows up reliably on lab testing, and why it deserves a careful, individualized approach rather than a one-size-fits-all supplement reflex.

This guide is part of Empire's Precision Nutrition resource cluster and is written for clinicians who want an accurate, practical understanding of vitamin D physiology, testing, and repletion. It is clinical education, not medical advice, and nothing here is a dosing protocol or a substitute for individualized clinical judgment.

How vitamin D is made and activated

The vitamin D pathway begins in the skin. When UVB rays from sunlight strike the skin, they convert a cholesterol precursor into pre-vitamin D. From there the molecule travels through a two-organ activation sequence: the liver converts the precursor into the circulating storage form (25-hydroxy-vitamin D), and the kidneys perform the final hydroxylation into the biologically active hormone, 1,25-dihydroxy-vitamin D. Understanding this chain matters clinically, because a problem anywhere along it — limited sun exposure, impaired liver or kidney function, or genetic variation in the activating enzymes — can leave a patient functionally short of vitamin D even when intake looks adequate on paper.

That last point is worth dwelling on. As Dr. Mark Tager teaches in Empire's Precision Nutrition course, genetics modulate this pathway in real ways: variation in the CYP2R1 enzyme can impair how efficiently a person converts vitamin D toward its active form, and variation in the GC gene — which codes for the vitamin D binding protein that carries vitamin D through the bloodstream to target cells — can mean some people transport less and therefore need somewhat more to get the same amount to the tissues that need it. This is one reason precision nutrition treats vitamin D as an individualized target rather than a fixed universal dose, a theme explored further in our overview of nutrigenomics and personalized nutrition.

Why deficiency is so common

If the body makes its own vitamin D, why is insufficiency so frequent? Because the modern environment quietly works against synthesis. We spend most of our days indoors and behind glass, which blocks UVB. Sunscreen — appropriate and important for skin-cancer prevention — also reduces the UVB that drives synthesis. Latitude and season limit usable sunlight for much of the year in much of the country. And several biological factors lower production directly.

• Skin tone. More melanin reduces vitamin D synthesis, so people with darker skin generally make less vitamin D from the same sun exposure.
• Aging skin. Older adults tend to have lower levels because aging skin loses some of its capacity to synthesize vitamin D — a meaningful consideration in any healthy-aging practice.
• Diet. The richest dietary sources are animal foods such as fatty fish and fortified dairy, so intake alone rarely closes the gap. National Institutes of Health survey data show that the majority of Americans don't even reach the estimated average requirement for vitamin D from their diets.

The net result is that low vitamin D is one of the more common findings clinicians actually measure, rather than a theoretical risk. That makes testing — rather than guessing — the right first move.

Testing: the 25-hydroxy-vitamin D test

The standard way to assess vitamin D status is the 25-hydroxy-vitamin D blood test. It measures the circulating storage form, which reflects both what the skin has synthesized and what the patient has taken in, making it the most reliable single snapshot of vitamin D status. (The active hormone, 1,25-dihydroxy-vitamin D, is tightly regulated by the body and can look normal even when stores are low, which is why it is not the right test for screening general status.)

Because vitamin D is so relevant to both skin and immune health, identifying insufficiency or lower-than-ideal levels is one of the more actionable findings a nutrition-focused work-up produces. Consumer-grade fingerstick options for vitamin D now exist alongside standard venous panels, expanding access to baseline screening. How to interpret a result, where target ranges sit, and how that interpretation should shift for an individual patient are part of the structured nutritional assessment and lab-testing framework Dr. Tager teaches in Empire's course — this guide deliberately stops short of a turnkey range-and-dose recipe.

What vitamin D does: skin, immunity, and well-being

Vitamin D's reputation rests on bone health, where its role in calcium handling is well established, but its relevance to a skin-and-aging practice runs broader.

Skin barrier and repair

Vitamin D helps maintain the skin's normal barrier function and supports tissue repair. Since the skin is both the site of synthesis and a target tissue, adequate vitamin D is part of the foundation for barrier integrity — one reason insufficiency is worth correcting in patients focused on skin quality and healthy aging. It is, importantly, one contributing factor among many; vitamin D is not a stand-alone treatment for any skin condition.

Immune function

Vitamin D plays a recognized role in immune regulation. Tager frames this vividly for patient conversations — expressing genuine surprise that over ninety percent of Americans don't meet the estimated average daily requirement for vitamin D, “so important for immunity.” That framing is a useful clinical entry point, but it should anchor to the same evidence-honest standard as everything else: support immunity by correcting a measured shortfall, not by chasing ever-higher numbers.

Mood and well-being

Vitamin D is also discussed in the context of mood and general well-being, and deficiencies in key micronutrients can affect mood, energy, and immune function. The honest clinical posture here is humility: associations exist, the evidence is uneven, and vitamin D is best positioned as one correctable input into well-being rather than a treatment for mood disorders.

Sun, diet, and supplements

There are three levers for vitamin D, and they are not interchangeable. Sunlight is the body's primary route but is constrained by season, latitude, skin tone, age, sunscreen, and the legitimate need to limit UV exposure for skin-cancer prevention — so it cannot be relied on as a prescription. Diet contributes through fatty fish, eggs, and fortified dairy, but as the survey data show, food alone rarely brings deficient patients to target. That leaves supplementation as the practical tool for closing a documented gap.

This is also where plant-based eating deserves attention. As Tager notes, vegans and vegetarians may have a harder time obtaining several key nutrients — including iron, calcium, zinc, B12, and vitamin D — precisely because the richest sources are animal foods. That doesn't make a plant-based diet inadequate; it makes testing and individualized supplementation more relevant for those patients. The broader landscape of vitamin and mineral status is covered in our guide to micronutrients: vitamins and minerals.

Correcting documented deficiency — the evidence-honest way

Here is the principle that should govern vitamin D repletion: treat the test, not the trend. The aim is to correct a deficiency that has been documented by measurement, then confirm with periodic retesting — not to put every patient on a high dose because vitamin D is popular. Tager's own stance is plain: very few people need or benefit from megadoses of vitamins, and in clinical practice it is far more common to see insufficiencies than toxicities — but toxicities do occur with excessive supplementation.

That caution is not theoretical, because vitamin D is fat-soluble. Unlike water-soluble vitamins such as B and C, which the body clears more readily, the fat-soluble vitamins A, D, E, and K are stored in the body and can accumulate. Chronic high-dose vitamin D can drive blood calcium to dangerous levels. This is why the responsible workflow is test → correct a documented deficiency → retest, and why Tager cautions clinicians who layer supplements onto a patient's existing regimen to be careful not to overdo the fat-soluble vitamins. More is not better.

Two further nuances matter and are taught, with their dosing detail, inside the course. First, vitamin D does not work alone — it interacts with cofactors such as magnesium and vitamin K2, which is part of why a single-nutrient megadose viewed in isolation is the wrong mental model. (We mention these cofactors because the biology is real, not to hand out a stacking protocol.) Second, certain common medications affect vitamin D status: acid-suppressing drugs and others can impair absorption of vitamin D and related nutrients, and some medications and chronic anxiety can increase its metabolism. The specific repletion ranges, retest cadence, cofactor strategy, and medication-interaction work-up are exactly the kind of individualized protocol Empire teaches in the nutraceutical supplementation module rather than something this page should prescribe.